As is known in the art, Microelectromechanical systems (MEMS) are integrated micro devices or systems combining electrical and mechanical components. MEMS devices may be fabricated using, for example, standard integrated circuit batch processing techniques. Exemplary applications for MEMS devices include sensing, controlling, and actuating on the micro scale. Such MEMS devices may function individually or in arrays to generate effects on a macro scale.
As is also known in the art, many MEMS devices require a vacuum environment in order to attain maximum performance. The vacuum package also provides protection and an optimal operating environment for the MEMS device. Specific examples of these MEMS devices include infrared MEMS such as bolometers, sometimes referred to as microbolometers, and certain inertial MEMS such as gyros and accelerometers. Previously, MEMS devices have been individually packaged in vacuum compatible packages after fabrication and dicing of the MEMS device. Often, however, the cost of packaging MEMS devices in traditional metal or ceramic packages may be on the order of about 10 to 100 times the device fabrication cost. This especially true if a vacuum is required in the package. These high packaging costs therefore make it difficult to develop commercially viable vacuum packaged MEMS devices. In addition, MEMS devices are fragile especially after dicing. Care must be taken in handling these devices, and traditional integrated circuit fabrication machinery cannot adequately handle and protect MEMS devices. Thus, special handling techniques have also been developed to protect the MEMS devices until vacuum packaging has been completed. These special handling procedures also add additional cost to the production of MEMS devices.
Over the years, various types of infrared detectors have been developed. Many include a substrate having thereon a focal plane array, the focal plane array including a plurality of detector elements that each correspond to a respective pixel. The substrate contains an integrated circuit which is electrically coupled to the detector elements, and which is commonly known as a read out integrated circuit (ROIC) and which is used to integrate the signal from each detector element and multiplex the signals off the chip with appropriate signal conditioning and processing.
Each detector element includes a membrane which is suspended at a location spaced above the top surface of the substrate, in order to facilitate thermal isolation. The membrane includes a thermally sensitive material, such as amorphous silicon (a-Si) or vanadium oxide (VOx). The membrane also includes two electrodes, which are each coupled to the thermally sensitive material, and which are also coupled to the ROIC in the substrate. As the temperature of the thermally sensitive material varies, the resistance of the thermally sensitive material also varies, and the ROIC in the substrate can determine the amount of thermal energy which has been received at a detector element by sensing the corresponding resistance change of that detector element.
As is the case with certain microelectromechanical (MEMS) devices, bolometers may need to be packaged in vacuum conditions for best performance. Exemplary requirements for the packaging of bolometer arrays include reliable hermetic sealing, the integration of IR window material with good infrared transmission, and high yield/low cost packaging. Both the reliability and the cost of MEMS devices depend upon encapsulation techniques chosen. For MEMS based bolometers, packaging may be done at the chip level or at the wafer level. A common way of packaging in this instance is to fabricate a protective, IR-transmitting cap wafer, or Window Cap Wafer (WCW), and bond it to an exposed surface of the semiconductor substrate, or device wafer, containing the active IR detector bolometer areas prior to dicing. The cap wafer, sometimes, also referred to as a cover or lid structure, is formed with cavities therein such that when the cap wafer is flipped and bonded to the device wafer, the cavities provide sufficient clearance to accommodate and protect the MEMS devices therein.
Uncooled infrared focal plane arrays operating at ambient temperature and without the use of active temperature stabilization require infrared optically blind reference pixels that do not absorb incident infrared radiation. These infrared optically blind reference pixels are used to determine ambient temperature of the focal plane which is required in the calibration of the focal plane array over the operating temperature of the focal plane array. This involves implementation of a gain and offset correction algorithm at any given temperature (sensed by the reference pixels) to the active detector elements in order to correct the image for ambient temperature drift effects, e.g., in an imaging focal plane array.
In the past, reference pixels have been made infrared optically blind by using short thermal isolation legs for the suspended infrared absorbing element in combination with placement of an infrared reflecting aluminum metallization directly on the reference pixel. However, reference pixels fabricated in this way have less Joule heating (e.g., in a voltage-biased suspended microbolometer structure) due to the shorter legs and to a lesser extent the added thermal mass of the aluminum reflector. As such, the output of the reference pixel as read out using a ROIC tends to diverge from the output of the infrared responsive suspended pixel detector elements, thereby limiting dynamic range both in terms of scene temperature and ambient temperature operating ranges.
As noted above, a cap wafer is formed with cavities therein such that when the cap wafer is flipped and bonded to the device wafer, the cavities provide sufficient clearance to accommodate and protect the MEMS devices therein. As also noted above, the cap is typically an infrared-transparent cover, or lid structure. These lids are often coated with an anti-reflective (AR) coating to reduce the reflective properties and increase the infrared transmission properties of the lid. In many applications, a WLP IR FPA package has a 300 um deep cavity with an ARC in the cap wafer to put the cap optical window as far from pixel plane as possible in order to minimize shadowing effect of optical defects in the image plane
As is also known in the art, some of these MEMS devices require antireflection coatings (ARCs).
As noted above, one such it detector is a bolometer. Bolometers are devices that act as thermal infrared (IR) sensors by absorbing electromagnetic radiation and thus increase their temperature. The resulting temperature increase is a function of the radiant energy striking the bolometer and is measured with for example, thermoelectric, pyroelectric, resistive or other temperature sensing principles. In the context of uncooled infrared imaging technologies, an infrared bolometer focal plane array (FPA) typically refers to resistive microbolometers, in which a temperature increase is measured by a corresponding resistance change. More specifically, a microbolometer is a type of resistor used as a detector in a thermal camera, for example. The microbolometer may include a tiny vanadium oxide or amorphous silicon resistor with a large temperature coefficient on a silicon element with large surface area, low heat capacity and good thermal isolation. Infrared radiation from a specific range of wavelengths strikes the vanadium oxide or amorphous silicon and changes its electrical resistance. Changes in scene temperature cause changes in the bolometer temperature, which are converted to electrical signals and processed into an image.
With regard to the ARC, a cavity, the floor of which becomes the inner surface of the window, is normally formed in the WCW to space the inside of the WCW away from the device wafer having the FPA detectors in order that defects in the ARC and window surface do not cast shadows on the FPA, which would appear as optical defects in the camera image. The cavity is formed with a Deep Reactive Ion Etch (DRIE). The DRIE cavity etching requires hours of etching per wafer. Controlling the window-detector gap can be accomplished by forming spacers between the device wafer and the WCW using layers of materials already employed in the fabrication of the detectors and package.
Today, in a wafer level packaged bolometer IR FPA, the window cap wafer (WCW) may have greater cost that the read-out IC wafer. Further, today, the WCW is typically a silicon-on-insulator (SOI) wafer. A substantial amount of the WCW cost is in the starting wafer (˜$400). Further, a Deep Reactive Ion Etch (DRIE) process is used to etch the cavity, with the oxide in the costly SOI wafer serving as an etch stop layer, and define the optical window at a cost of ˜$200, and the anti-reflective coating (ARC) on the inner surface costs about $500 per wafer.